Hi.
City Building is a type of game where players are building cities / focusing on the visuals instead of wars.
In EE2 City Building was usually played together with Role Play - it means players were building nice cities and had planned wars / battles.

In Unofficial Patch 1.5 all new City Builder buildings/units are unlocked by default. Everything you need to do is produce a City Builder (or Gardener) in the City Center. City Builder buildings are usually expensive and have no additional effects in game (to not ruin the original game balance and prevent abuses in normal Multiplayer games).